The Baby Boomer/Millennial housing mismatch is well known: As Boomers age, an upcoming glut of suburban and exurban homes will stand empty and unwanted, leaving both generations at a loss. Downsizing empty-nesters won’t find buyers, because Millennials want smaller homes or condos in or nearer to the city, not big four-bedroom Colonials with yards. And younger adults won’t be able to afford such single-family abodes because urban housing has become too pricey.

There’s another angle to this story, and it implicates Boomers in the priciness. Randy Shaw, homeless advocate and director of San Francisco’s Tenderloin Housing Clinic, has penned a new book, Generation Priced Out: Who Gets to Live in the New Urban America, that examines how urban Boomer homeowners, in their quest to fend off “density” (and apartment renters) in their neighborhoods, have consistently—and incredibly successfully—blocked the construction of affordable housing. Such NIMBYism has hindered the building of more housing in areas desirable to Millennials, keeping stock low, prices high, and younger, less-affluent residents—as well as the working and middle classes more broadly—out.

“Discussions around a lack of affordable urban housing often focus on developers and speculators as the villains, but homeowner opposition to new apartments is a large part of the problem,” said Shaw.

CityLab recently spoke with Shaw about how this phenomenon is taking place in the country’s most liberal-leaning cities, how Millennials are fighting it, and the reasons some Boomers are starting to change their outlook.

You’ve spent decades as a housing advocate for the homeless. What made you write a book on the working and middle classes?

I was so struck by the 2016 Ghost Ship fire in Oakland that killed 36 people when they became trapped in a warehouse that had been informally turned into an artist collective and residence. Oakland used to be the affordable alternative to San Francisco, but now people with fewer means can’t even afford habitable housing there.

Our bluest cities, like San Francisco, are the ones pricing out the working and middle classes. This is the opposite of what these cities espouse. This isn’t just happening in historically expensive places like San Francisco. Austin and Seattle, though still more affordable, have seen their housing prices skyrocket. Seattle has had a 155 percent raise in rents in the last 10 years.

The bottom line is they’re not building enough. Though it’s often said that “we can’t build our way of the housing crisis,” if you don’t build the units, there’s no possible way to address affordability. If you look at Seattle, which builds double the housing of San Francisco, the city is starting to see rents and home prices slow. Of course, just building housing isn’t enough. If a city only builds upscale units, there’s nothing for working people.

It’s also important to build in neighborhoods that are already gentrified. These high-opportunity neighborhoods must serve more economically diverse residents, and cities that claim to promote inclusion cannot just relegate the non-rich to economically segregated parts of town.

Where do Baby Boomers come into this?

A recent national study found that from 1983 to 2013 housing wealth increased almost entirely among America’s oldest and wealthiest residents. Urban Boomer homeowners are part of this trend, and they’ve made enormous profits by working to restrict housing supply where they live. Neighborhood councils and homeowners associations are usually comprised of white, wealthier Boomers, even when the neighborhoods are more diverse in terms of race, class, and age. They use their position in these organizations to impede new building such as fourplexes or triplexes. Even a neighborhood like Los Angeles’s Venice, which has a reputation for being bohemian and progressive, doesn’t build much affordable housing.

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But Millennials are starting to organize and push against this trend. They’re challenging zoning laws and pushing for more housing in cities like Berkeley, Cambridge, Portland, and Minneapolis. They’ve become a real presence at city council meetings, which have long been dominated by Boomer opponents of housing. I think they’re a very talented group—they have to be to overcome the obstacles.

Is there any way to get older homeowners to work with Millennials on this issue?

Environmentalism is the core connector. As people of all ages work for environmental sustainability, they understand that we need to get people out of cars, and this means getting as many people as possible to live in or close to cities and use public transport. And that means making those areas more affordable. A lot of urban Boomers are also seeing that their children and grandchildren can’t afford to live near them. This element of self-interest is starting to turn the tide a bit. I do think the Millennial arguments for density and affordable housing are winning, but we need to speed up the process.

I point out the HALA program because many states bar inclusionary zoning. In San Francisco we require a percentage of units in a new building to be rented or sold at below market rate, but this isn’t the case in most places. In such circumstances, it’s hard to curtail purely market rate housing. The HALA program supplies density bonuses, meaning that it gives developers the option to build more floors in exchange for including more affordable units. Cities across the country can implement these bonuses regardless of their states’ rules on zoning.

There are a number of other key strategies for high-rent cities, such as using publicly owned land for affordable housing and funding the nonprofit purchase of apartment buildings in neighborhoods facing displacement and gentrification.

What about the role of the federal government?

In 2000, I wrote an article about how housing wasn’t being covered in the presidential race. I could have simply republished it during the 2004, 2008, 2012, and 2016 elections. The federal government’s disregard for affordable housing has created so many homeless and a crisis of family housing. We need to fuse the federal and the local and private housing push and make it a national campaign to deal with the affordability crisis. I’m heartened that Senators Elizabeth Warren, Cory Booker, and Kamala Harris have put some focus on housing recently.

But in the meantime, localities can build thousands of units that will make a difference. There’s nothing stopping us; it’s just political will. Cities can make these changes now.